diff --git a/lib/navigationlib.php b/lib/navigationlib.php
index 3fe6df05ff7..20f96594b2f 100644
--- a/lib/navigationlib.php
+++ b/lib/navigationlib.php
@@ -1633,6 +1633,7 @@ class global_navigation extends navigation_node {
         $activities = array();
 
         foreach ($sections as $key => $section) {
+            // Clone and unset summary to prevent $SESSION bloat (MDL-31802).
             $sections[$key] = clone($section);
             unset($sections[$key]->summary);
             $sections[$key]->hasactivites = false;